1. Field of the Invention
This invention relates to electrodes for use in measuring an ionic specie concentration. More specifically, this invention relates to a chemically treated graphite electrode that is suitable for use as an external reference electrode, that can be modified as an internal reference electrode for use in an ion-selective electrode.
2. Prior Art
To measure the concentration, or more correctly, the activity of an ion, an ion-selective electrode and an external reference electrode are utilized. A voltage potential or emf developed between these two electrodes can be detected with a potentiometric device. A change in potential between the electrodes will be proportional to the logarithm of the activity of the ion, as described by the well-known Nikolski-Eisenman equation.
The construction of an ion-selective electrode (ISE) in the past has involved an electrode body, usually consisting of a glass or plastic tube. Such electrode body has been filled with an aqueous reference electrolyte solution, for example, calcium chloride for a calcium ISE. In contact with the reference solution at one end, is a reversible half-cell that consists of a silver wire coated with silver chloride. The other end of the electrode body also in contact with the reference solution is covered with an ion-selective membrane. Examples of these types of electrodes are shown in U.S. Pat. Nos. 3,598,713; 3,502,560; 3,562,129; 3,691,047 and 3,753,887. A U.S. Pat. No. 4,214,968 dated July 29, 1980, to Battaglia, et al., summarizes the disadvantages of these types of ion-selective electrodes; for example, high cost, low durability and poor reproducibility.
Attempts to overcome the above shortcomings led to the "coated wire" ion-selective electrode illustrated in an article by Cattrall, R. W., and Frieser, H., Analytical Chemistry 43: 1905 and 1906 (1971). This electrode consisted of a platinum wire coated at one end with a layer of an ion-sensor mixed in polyvinylchloride (PVC). This type of electrode does not contain a defined internal reference electrode or an internal reference solution. Although simple and inexpensive, such an electrode drifts, needs frequent recalibration and is commercially unattractive, as set out in the Battaglia, et al patent, U.S. Pat. No. 4,214,968.
A hybrid reference electrode that incorporates features of a conventional liquid junction electrode and the coated wire electrode was developed by Ruzicka and Lamm, U.S. Pat. No. 3,926,764, dated Dec. 16, 1975. This electrode, rather than using a metal wire as the conductor, utilized a mixture of powdered carbon and teflon pressed or sintered together to form a solid conducting hydrophobic substrate. Embedded in one end of the teflon/graphite substrate was an "electrochemically reversible" redox system such as calomel (Hg/HgCl.sub.2). The redox paste was in contact with a humid (10% H.sub.2 O) layer of water-soluble salt, such as CaCl.sub.2 for the Ca.sup.2+ ion-selective electrode. In some cases, as described in an article by J. Ruzicka, E. H. Hansen, J. Tjell Analytical Chimica Acta 67: 155-178 (1973), the water-soluble electrolyte is included in the calomel paste as CaSO.sub.4. When the above system is coated with a hydrophobic membrane, for example a PVC membrane containing an ion-exchanger or carrier appropriate for a particular ion the electrode will constitute an ion sensitive electrode. Electrodes of this type generally drift only about 5 mV/day, according to Hulanicki and Trojanowicz Analytica Chimica Acta 87: 411-417 (1976). As such, they represent a real improvement over "virgin" graphite internal reference electrodes which are not hydrophobized and do not contain an electrically reversible redox system as described by Ansaldi, A. and Epstein, S., Analytical Chemistry 45: 595-596 (1973); the "virgin" graphite electrodes typically drift 600 mV/Day as set out in Fleet, B., Bound, G. P. and Sandbach, D. R. Bioelectrochemistry Bioenergetics 3: 158-168 (1976).
It is noteworthy that there was no substantial difference in drift when the calomel paste and dissociable salt was omitted from the teflon/graphite electrode, as set out in FIG. 1 of A. Hulanicki and M. Trojanowicz, Analytica Chimica Acta 87: 411-417 (1976). The significance of the observation was not noted in this work or in any subsequent work to the knowledge of the present inventors prior to the present application. This is evidenced by the considerable efforts by inventors to stabilize electrode drift by establishing an electrochemically defined reference junction. One such effort was to apply a hydrophilic gel to the conducting substrate (metal) to form a "solid-state electrode", as shown in U.S. Pat. No. 3,833,495, by Grubb dated Sept. 3, 1974 and U.S. Pat. No. 3,671,414, dated June 20, 1972; a French Pat. No. 2,158,905, dated June 15, 1973 and a patent by Genshaw, et al., U.S. Pat. No. 3,856,649, dated Dec. 24, 1974. The major drawback of this type of electrode is that it must be preconditioned by a technique that hydrates the electrode or else the electrode will yield non-Nernstian responses and drift substantially, as disccused in the Battaglia, et al. patent U.S. Pat. No. 4,214,968. Also, these electrodes tend to become inoperative after a short time since the membranes blister or swell as described by B. Fleet, G. P. Bound and D. Sandbach. Bioelectrochemistry and Bioenergetics 3: 158-168 (1976).
An attempt to correct the problems inherent in a hydrated gel internal reference electrode is shown in the Battaglia et al. patent. In this patent, the gelled internal reference solution is "dried" prior to application of the ion-sensitive membrane, resulting in "instant" working electrodes requiring no preconditioning:
" . . . the drift exhibited by these electrodes although sometimes substantial, can be calibrated to provide accurate and reproducible determinations of the concentration of specific ions in test solutions"
FIG. 3 of the Battaglia et al. patent shows drifts at the rate of about 60 mV/hr. This patent provides two general types of configurations for the electrode, as shown in FIGS. 1 and 2, and in example 2 therein, that use carbon as the conductive layer in contact with a redox couple layer.
The above cited patents all teach that, in order to obtain stable reference electrode systems, an electrochemically defined reversible system is required in contact with a conducting substrate, especially if that substrate is carbon or graphite. The literature emphasizes that carbon by itself displays considerable drift B. Fleet, G. P. Bound and D. Sanbach, 1976. Even though prior patents and the scientific literature have focused on the chemically defined redox system, it has been observed, as set out hereinabove, Hulanicki and Trojanowicz in Analytica Chimica Acta 87 411-417 (1976), that a teflon/graphite substrate produces reference electrode with drift comparable to defined electrochemical systems. Thus, although the emphasis has been on pastes, gels, dried gels, etc., to stabilize performance, there is at least this one indication to support the present inventors' observations that effective hydrophobization of a carbon substrate will result in reference electrode stability. The present inventors, however, arrive at their hydrophobic procedure not by a physical mixing of a hydrophobic material (e.g. teflon) with carbon, as done by Ruzicka and Lamm, U.S. Pat. No. 3,926,764, but by chemicaly treating the carbon to render the surface hydrophobic.